Devices having a power supply that is operated by battery or rechargeable battery often operate only within discrete activity phases and bridge the times in between with power-saving phases within which many or virtually all of the components are operated without power or a clock. In the case of a microprocessor in such a device, one refers to the sleep state in the power-saving phase and to the awake state in the activity phase. If a device has a plurality of processors which are each responsible for particular functions that are separate from one another, the sleep and awake periods of the processors are, in principle, independent of one another and each processor can independently change from the awake state to the sleep state and vice versa. One example of this is a mobile radio telephone having a modem processor as an interface to the mobile radio network and an application processor as an interface to the user.
Since two processors can communicate only when both processors are in the awake state, a V.24 interface having four additional lines which are used to control the sleep and awake states is used, for example, in conventional devices (which are operated by battery or rechargeable battery) for communication between the processors. Each processor contains an additional output line, which it can use to indicate its state to the other processor, and a further output line which it can use to wake up the other processor from the sleep state. The disadvantage of this is that additional lines in the device result in a larger layout and in a greater number of layers of printed circuit boards and in larger plug connectors.